The development of seismic anisotropy below south-central Alaska: evidence from local earthquake shear wave splitting

Abstract

Summary The Transportable Array in south-central Alaska spans several subduction zone features: backarc, forearc and volcanic arc, making it an ideal tool to study subduction zone anisotropy. Shear-wave splitting analysis of 157 local earthquakes of mb≥3.0 that occurred between 2014 and 2019 yields 210 high quality measurements at 23 stations. Splitting delay times (δt) are generally small (δt≈0.3 s), increasing with distance from the trench. Arc parallel fast directions, φ, are only seen in the forearc, but rotate to arc perpendicular φ in the backarc. Observed φ values generally do not parallel teleseismic SKS splitting results, implying the latter is sensitive primarily to sub-slab mantle flow, not mantle wedge dynamics. The forearc local-earthquake signal likely originates from anisotropic serpentinite in fractures atop the subducting Pacific plate, with possible additional signal coming from fractures in the North American crust. Mantle wedge corner flow, potentially with additional arc-perpendicular anisotropy in the subducting slab, explains backarc anisotropy.

Anisotropy Variations in the Alaska Subduction Zone Based on Shear‐Wave Splitting From Intraslab Earthquakes

“…(1999) and Karlowska et al. (2021) also show an arc‐parallel local splitting pattern in region L2 as well as evidence of the sharp change in fast direction across the arc. The ray paths that sample the cold part of the mantle wedge all originate in the slab and have some path length in the slab.…”

“…(1999) and Karlowska et al. (2021) and provide improved data coverage for this region. The large splitting delay times for local ray paths that mainly sample slab indicate that the subducting Pacific lithosphere contains significant anisotropy. Both local S and SKS datasets show plate convergence fast directions at the Kenai Peninsula where there is no underlying mantle wedge.…”

“…Karlowska et al. (2021) used 23 Transportable Array (TA) stations and observed arc‐parallel fast directions in the forearc transitioning to arc perpendicular in the backarc. Wiemer et al.…”

“…Although there are exceptions (Hammond et al, 2010;Schlaphorst et al, 2017), most local splitting studies around the globe have suggested that the mantle wedge is the main anisotropic structure in the subduction system (Abt et al, 2009;León Soto & Valenzuela, 2013;Long & van der Hilst, 2006;Nakajima & Hasegawa, 2004). In the Alaska subduction zone specifically, anisotropy has been suggested to be present in the mantle wedge and in the subducting Pacific lithosphere and subslab asthenosphere (Christensen & Abers, 2010;Hanna & Long, 2012;Karlowska et al, 2021;McPherson et al, 2020;Perttu et al, 2014;Song & Kawakatsu, 2013;Venereau et al, 2019 (Argus et al, 2011). Red lines denote active faults (Koehler et al, 2012).…”

“…There have been few local splitting studies in Alaska, they all lack dense station coverage in the backarc, and they show a range of different splitting patterns for various sections of the subduction zone. Karlowska et al (2021) used 23 Transportable Array (TA) stations and observed arc-parallel fast directions in the forearc transitioning to arc perpendicular in the backarc. Wiemer et al (1999) used three stations and showed arc-parallel fast directions in the forearc, along with several arc-perpendicular measurements in the backarc near Redoubt volcano.…”